There are two types of stable nitrogen isotopes, namely 14N and 15N, with the former being present in the natural world at 99.635 atom % and the latter being present at 0.365 atom %. In the present invention, a heavy nitrogen isotope refers to 15N.
Nitrogen in the atmosphere (molecular nitrogen: N2) consists of three types of isotope molecules, namely 14N2, 14N15N and 15N2, and their respective prevalence in the atmosphere is as indicated below.
14N2: 99.635 atom %×99.635 atom %=99.271 mol %
14N15N: 99.635 atom %×0.365 atom %×2=0.727 mol %
15N2: 0.365 atom %×0.365 atom %=0.001 mol %
The stable nitrogen isotope, 15N, is currently used mainly as a tracer in the fields of natural science and medicine. In addition, the use of 15N has also recently been examined in the field of energy as well.
Since 15N has an extremely low prevalence in nature, there is a need for a method that allows it to be efficiently concentrated to high concentrations.
Examples of methods for concentrating 15N include a chemical exchange method that utilizes the phenomenon by which the equilibrium distribution concentration of an isotope differs according to the particular compound, a vapor phase adsorption method that utilizes differences in the adsorption behavior of ammonia to zeolite attributable to the isotope effect, and a method involving concentration of 15N18O to obtain 15N and 18O by low-temperature distillation of nitric oxide (NO).
Methods for concentrating isotopes by low-temperature distillation include distillation of nitric oxide as described above, and carbon monoxide distillation and methane distillation for the purpose of concentrating 13C (Japanese Unexamined Patent No. 188240, Japanese Unexamined Patent No. 210945). In addition, 18O can be concentrated to a high concentration by combining oxygen distillation and isotope scrambling (WO 00/27509).
On the other hand, although examples of methods for concentrating 15N to high concentrations include chemical exchange method, vapor phase adsorption method and nitric oxide (NO) distillation method, a method that combines low-temperature distillation of nitrogen (N2) and isotope scrambling is the most promising for large-volume production with high purity.
Chemical exchange method and vapor phase adsorption method are not suitable for large-volume production on an industrial scale. In addition, it is ultimately not easy to obtain highly pure N2 free of impurities other than N2 since isotope separation is mediated by nitrogen compounds.
Nitric oxide distillation has similar problems since oxygen molecules must be separated in a subsequent step. Moreover, since nitric oxide is both toxic and corrosive, it also has the problem of resulting in difficult handling.
Although concentration by low-temperature distillation method is promising for large-volume production, this method has the problems indicated below in the case concentrating 15N to a final isotope concentration of 50 atom % or higher.
The composition of isotope molecules of nitrogen consists of 14N2 at 99.272 mol %, 14N15N at 0.727 mol % and 15N2 at 13.3 mol-ppm. Although it is possible to concentrate 14N15N nitrogen molecules containing 50% 15N provided they are concentrated to no higher than about 50 atom %, it is not easy to concentrate 15N to 80 atom %. This is because, in this case, 15N2 must be concentrated to at least 60 mol % (with the remaining 40 mol % assumed to be 14N15N), and even if 15N2 nitrogen molecules were concentrated at a yield of 100%, for example, this would require 45000 times or more raw material than the amount of the final product.
In order to concentrate 15N to 99 atom %, 15N2 must be concentrated to at least 98 mol % (with the remaining 2 mol % assumed to be 14N15N), thus requiring nearly 74,000 times more raw material than the amount of the final product even if the yield of 15N2 was 100%.
In this case, since a large amount of 14N15N is discarded, the yield of 15N is only 0.368%, thus making this inefficient as an isotope separation process as well.
It is unrealistic to assume a value of 100% for the yield of 15N2, and in actuality, roughly 10 times or more of raw material is considered to be required. Thus, the yield of 15N is one-tenth that or less.
In this manner, in the case of concentrating 15N by low-temperature distillation of the diatomic molecule of nitrogen, N2, the level of difficulty is completely different depending on whether the final isotope concentration is to be 50 atom % or more, or 50 atom % or less. If the final isotope concentration is only required to be 50 atom % or less, although this can be accomplished simply by concentrating 14N15N that is present in comparatively abundant amounts, if the final isotope concentration is to be 50 atom % or more, it is necessary to concentrate 15N2, which is only present in extremely small amounts, thereby requiring a large amount of raw material.
Moreover, not only is a large amount of raw material required if the final isotope concentration is to be 99 atom %, since nearly all of the comparatively abundant 14N15N is discarded, the process has extremely poor efficiency. Thus, it is realistically impossible to obtain 15N at a concentration of, for example, 80 atom % or more by low-temperature distillation of nitrogen alone.
As a means of solving such problems, the applicant has previously filed a patent application for a method that combines nitrogen distillation and isotope scrambling as described in International Publication No. WO 2007/1228934.